Memory

Memory on the WRT54GL is still (on the whole) not fully understood, this page is dedicated to rectifying that problem. View the talk page for a simple (possibly outdated) dump of what memory should look like.

Introduction

Memory on almost every MIPS processor is broken into two major sections---user memory and kernel memory. Each of these sections occupies two gigabytes of memory and provides a simple mapping from code addresses to physical addresses which will be explained later.

User Memory

User memory, also known as KUSEG, ranges from 0x0000 0000 to 0x7FFF FFFF and can only be accessed when the processor is running in user-mode or (more dangerously) if the error level bit in the status register of co-processor 0 is set. While executing in kernel-mode or supervisor-mode memory references to this segment will result in TLB exceptions.

Since the WRT54GL routers only have 16 megabytes of RAM, it is currently assumed that 0x0000 0000--0x00FF FFFF has a 1-1 mapping with physical memory.

Kernel Memory

Kernel memory occupies the processor memory range from 0x8000 0000 to 0xFFFF FFFF and is sub-divided into 3 smaller regions. These regions are:

• KSEG0 -- unmapped, cached memory (512 megabytes, starting at 0x8000 0000),
• KSEG1 -- unmapped, uncached memory (512 megabytes, starting at 0xA000 0000),
• KSEG2 -- mapped memory (1 gigabytes, starting at 0xC000 0000).

It is important to notice that all direct memory accesses (DMAs) occur in the unmapped, uncached region of memory since devices often produce volatile data which would be dangerous to cache.

Since physical memory is shared between user and kernel processes the mapping for KUSEG is simply 1-1, KSEG0 is address - 0x8000 0000, KSEG1 is address - 0xA000 0000, and finally KSEG2 is address - 0xC000 0000.

User Segment

As discussed above the user segment of memory occupies addresses from 0x0000 0000 to 0x7FFF FFFF. Since physical memory is shared between each of the segments it should be noted that the first few pages of the user segment will actually be kernel code and should not be writable to user-level processes. Also the NULL pointer (0x0000 0000) could be considered a valid pointer in user-mode and this should be avoided so NULL pointer references are not ignored.

Embedded XINU does not currently have support for user-mode operations, so not much is known about the user segment.

Kernel Segment 0

This segment exists from memory address 0x8000 0000 to 0x9FFF FFFF and is cached but remains unmapped (beyond the simple address - 0x8000 0000 mapping).

The first 0x1000 bytes of this segment are reserved system space and contains small amounts of code for the interrupt handler. When an interrupt or exception occurs, the MIPS processor will jump to code located at 0x8000 0180, expecting to find handler code at that location. For safety reasons that code can only consume 0x20 bytes of memory (enough to jump to a safer, more robust handler). Embedded XINU takes advantage of the space immediately after the end of interrupt handling code by loading entry vectors for the various interrupts and exceptions for speedy lookup.

After the reserved system space it is safe to load generic code, XINU code is loaded to 0x8000 1000 and begins execution at the same offset. XINU code begins with the text segment, followed by the data and BSS segments. After the compiled image is loaded into RAM, XINU allocates a specific amount of memory for the kernel stack immediately after the BSS segment. Once the kernel stack has been setup, XINU initializes the heap beginning directly above the kernel stack and continuing until the end of physical memory (0x80FF FFFF).

Kernel memory allocation will take memory addresses from the heap initialized in this segment.

References

Sweetman, Dominic. See MIPS Run. San Francisco: Morgan Kaufmann Publishers, 2007.